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SubjectRe: [PATCH] sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters
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Peter Xu <peterx@redhat.com> writes:

> On Wed, Apr 01, 2020 at 10:30:08PM +0200, Thomas Gleixner wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> > @@ -169,8 +169,12 @@ static int __init housekeeping_isolcpus_setup(char *str)
>> > continue;
>> > }
>> >
>> > - pr_warn("isolcpus: Error, unknown flag\n");
>> > - return 0;
>> > + str = strchr(str, ',');
>> > + if (str)
>> > + /* Skip unknown sub-parameter */
>> > + str++;
>> > + else
>> > + return 0;
>>
>> Just looked at it again because I wanted to apply this and contrary to
>> last time I figured out that this is broken:
>>
>> isolcpus=nohz,domain1,3,5
>>
>> is a malformatted option, but the above will make it "valid" and result
>> in:
>>
>> HK_FLAG_TICK and a cpumask of 3,5.
>
> I would think this is no worse than applying nothing - I read the
> first "isalpha()" check as something like "the subparameter's first
> character must not be a digit", so to differenciate with the cpu list.
> If we keep this, we can still have subparams like "double-word".

It _is_ worse. If the intention is to write 'nohz,domain,1,3,5' and
that missing comma morphs it silently into 'nohz,3,5' then this is
really a step backwards. The upstream version would tell you that you
screwed up.

>> static int __init housekeeping_isolcpus_setup(char *str)
>> {
>> unsigned int flags = 0;
>> + char *par;
>> + int len;
>>
>> while (isalpha(*str)) {
>> if (!strncmp(str, "nohz,", 5)) {
>> @@ -169,8 +171,17 @@ static int __init housekeeping_isolcpus_
>> continue;
>> }
>>
>> - pr_warn("isolcpus: Error, unknown flag\n");
>> - return 0;
>> + /*
>> + * Skip unknown sub-parameter and validate that it is not
>> + * containing an invalid character.
>> + */
>> + for (par = str, len = 0; isalpha(*str); str++, len++);
>> + if (*str != ',') {
>> + pr_warn("isolcpus: Invalid flag %*s\n", len, par);
>
> ... this will dump "isolcpus: Invalid flag domain1,3,5", is this what
> we wanted? Maybe only dumps "domain1"?

No, it will dump: "domain1" at least if my understanding of is_alpha()
and the '%*s' format option is halfways correct

> For me so far I would still prefer the original one, giving more
> freedom to the future params and the patch is also a bit easier (but I

Again. It does not matter whether the patch is easier or not. What
matters is correctness and usability. Silently converting a typo into
something else is horrible at best.

> definitely like the pr_warn when there's unknown subparams). But just
> let me know your preference and I'll follow yours when repost.

Enforcing a pure 'is_alpha()' subparam space is not really a substantial
restriction. Feel free to extend it by adding '|| *str == '_' if you
really think that provides a value.

Thanks,

tglx

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